Development of a conjugated diene rubber suitable for use in a rubber material (rubber composition) for tires exhibiting reduced rolling resistance, high abrasion resistance, excellent breaking strength, as well as superior operational stability performance represented by wet skid resistance is desired to cope with a recent demand for vehicles with a low fuel consumption.
A method of using a rubber composition containing silica or a mixture of silica and carbon black as a reinforcing agent as a rubber material for tires has been proposed. Tire treads containing silica or a mixture of silica and carbon black have small rolling resistance and exhibit excellent operational stability performance represented by wet skid resistance. However, a rubber composition in which silica is blended is generally interior in processability to rubber compositions in which carbon black is blended.
As a related art, there is disclosed a method for producing a conjugated diene copolymer rubber, where a conjugated diene copolymer rubber is obtained by copolymerizing a conjugated diene compound and an aromatic vinyl compound, adding the conjugated diene compound to the reaction system further to react with the copolymer, and then causing an appropriate coupling agent to react with active copolymer terminals (for example, refer to Patent Documents 1 and 2). A conjugated diene copolymer rubber suitable for obtaining a rubber composition with a small rolling resistance can easily be produced by this method.
However, the conjugated diene copolymer rubber obtained by the method disclosed in Patent Document 1 does not necessarily have excellent processability. Therefore, it has been necessary to improve processability of the resulting rubber. In addition, in the case that the resultant rubber is vulcanized since the rubber after the vulcanization (vulcanized rubber) tends to decrease in hardness, development of a production method for improving the hardness has been desired. Further, in order to reduce the production cost while avoiding complication of production steps, development of a process that can complete polymerization for a time equal to that for the conventional process, without prolonging the time required for polymerization has been desired.    [Patent Document 1] JP-A-2003-171418    [Patent Document 2] JP-A-2004-51869